1. Field of the Invention
The present invention relates to an exhaust-gas purifying apparatus.
2. Description of the Related Art
In order to purify exhaust gases emitted from automobiles and motorcycles, exhaust-gas purifying apparatuses have been used. There are many types of exhaust-gas purifying apparatuses such as thermal reactor-system, lean burning-system, engine modification-system, and catalyst-system exhaust-gas purifying apparatuses. Among them, catalyst-system exhaust-gas purifying apparatuses have been used extensively.
Catalyst-system exhaust-gas purifying apparatuses purify exhaust gases by using catalytic precious metals such as Pt, Rh and Pd. In catalyst-system exhaust-gas purifying apparatuses, exhaust-gas purifying catalysts are used which are made in the following manner. A catalytic layer is formed on a surface of a catalyst support with activated alumina such as γ-alumina. Then, one or more catalytic precious metals are loaded on the catalytic layer.
As for the materials of catalyst supports, heat-resistant materials are used because catalyst supports are exposed to high-temperature exhaust gases. As such materials, it is possible to name ceramics, such as cordierite, heat-resistant metals, such as stainless steels.
Catalyst supports made of ceramics suffer from such disadvantages that they are susceptible to mechanical shocks and exhibit large emission resistance. Accordingly, catalyst supports made of metals have come to be used due to the reasons that the pressure loss of exhaust systems should be reduced or the heat resistance of catalyst supports should be improved.
An exhaust-gas purifying apparatus provided with a metallic catalyst support can be manufactured in the following manner, for example. A steel stock is rolled as a foil-shaped or sheet-shaped workpiece. The steel stock can be SUS304 (as per Japanese Industrial Standard (JIS), i.e., 18Cr-8Ni austenic stainless steel), or SUS430 (as per JIS, i.e., 16Cr ferritic stainless steel). The resulting foil-shaped or sheet-shaped workpiece is processed into a metallic catalyst support. Then, a catalytic layer is formed on a surface of the resultant metallic catalyst support. Finally, one or more catalytic precious metals are loaded on the catalytic layer. Thus, an exhaust-gas purifying apparatus provided with a metallic catalyst support is completed.
Depending on the shapes of catalyst supports, exhaust-gas purifying apparatuses can be divided into monolithic, granular, honeycomb-shaped and pipe-shaped exhaust-gas purifying apparatuses.
In honeycomb-shaped exhaust-gas purifying apparatuses, there is a problem in that metallic catalyst supports might be melted by misfires which are transmitted from internal combustion engines. Specifically, when metallic catalyst supports are melted, the effective loading amount of catalytic precious metals might be decreased, or honeycomb-shaped cells might be clogged to lower the exhaust-gas purifying ability of honeycomb-shaped exhaust-gas purifying apparatuses.
Moreover, in pipe-shaped exhaust-gas purifying apparatuses, the axial length should be prolonged in order to secure a desirable exhaust-gas purifying ability. Accordingly, the boardability problem might associate with pipe-shaped exhaust-gas purifying apparatuses. In addition, when the axial length of pipe-shaped exhaust-purifying apparatuses is prolonged, the exhaust-gas temperature might be dropped to lower the exhaust-gas purifying ability of pipe-shaped exhaust-gas purifying apparatuses.
Consequently, pipe-shaped exhaust-gas purifying apparatuses have been developed whose axial length is shortened. For example, Japanese Unexamined Patent Publication (KOKAI) No. 9-228,832 and Japanese Unexamined Patent Publication (KOKAI) No. 9-317,452 propose such a pipe-shaped exhaust-gas purifying apparatus.
Japanese Unexamined Patent Publication (KOKAI) No. 9-228,832 discloses a catalytic converter in which a metallic support, manufactured by winding a corrugated metallic plate, is fitted into a mantle.
Japanese Unexamined Patent Publication (KOKAI) No. 9-317,452 discloses an exhaust-gas purifying apparatus in which a plurality of minor-diameter pipes are disposed in a major-diameter pipe.
However, the catalyst converter and exhaust-gas purifying apparatus disclosed in the publications have a problem in that exhaust gases passing through the mantle and major-diameter pipe do not fully contact with the metallic support and minor-diameter pipes. Specifically, their exhaust-gas purifying abilities are insufficient. To be more precise, since the metallic support and minor-diameter pipes are disposed along the flow of exhaust gases, exhaust gases are less like to contact with the catalyst loaded on the metallic support and minor-diameter pipes when they pass through the inside of the mantle and major-diameter pipe.